Heretofore, a physician has comprehensively carried out a diagnosis about a ventilatory function of lungs by both pieces of information coming from an eye and an ear, such as image information by a chest X-ray film (an image imaged when a lung field becomes maximum at the time of deep breathing) and sound information of a stethoscope.
As opposed to this, in recent years, an attempt has been being made, which is to image a dynamic image of a chest portion by using a semiconductor image sensor such as an FPD (flat panel detector), and to apply the dynamic image to the diagnosis. For example, in Patent Literature 1, there is described a technology for generating difference images showing differences of signal values among a plurality of image frames which compose the dynamic image, and obtaining maximum values of the respective signal values from the generated difference images to display the obtained image.